Covalent attachment of the hydrophilic polymer, poly(ethylene glycol), abbreviated PEG, also known as poly(ethylene oxide), abbreviated PEO, to molecules and surfaces is of considerable utility in biotechnology and medicine. In its most common form, PEG is a linear polymer terminated at each end with hydroxyl groups:HO—CH2CH2O—(CH2CH2O)n—CH2CH2—OH
The above polymer, alpha-,omega-dihydroxylpoly(ethylene glycol), can be represented in brief form as “HO—PEG-OH” where it is understood that the “PEG” symbol represents the following structural unit:—CH2CH2O—(CH2CH2O)n—CH2CH2—where n typically ranges from about 3 to about 4000.
PEG is commonly used as methoxy-PEG-OH, or mPEG in brief, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification. The structure of mPEG is given below.CH3O—(CH2CH2O)n—CH2CH2—OHmPEG
Random or block copolymers of ethylene oxide and propylene oxide, shown below, are closely related to PEG in their chemistry, and they can be substituted for PEG in many of its applications:HO—CH2CHRO(CH2CHRO)nCH2CHR—OHwherein each R is independently H or CH3.
PEG is a polymer having the properties of solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule “conjugate” soluble. For example, it has been shown that the water-insoluble drug paclitaxel, when coupled to PEG, becomes water-soluble. Greenwald, et al., J. Org. Chem., 60:331-336 (1995).
To couple PEG to a molecule, such as a protein, it is often necessary to “activate” the PEG by preparing a derivative of the PEG having a functional group at a terminus thereof. The functional group can react with certain moieties on the protein, such as an amino group, thus forming a PEG-protein conjugate.
In U.S. Pat. No. 5,650,234, which is incorporated by reference herein in its entirety, a 1-benzotriazolylcarbonate ester of poly(ethylene glycol) is described. The multi-step process described in the '234 patent for forming the 1-benzotriazolylcarbonate ester of PEG includes reaction of a PEG molecule with the volatile and hazardous compound, phosgene, in order to form a PEG chloroformate intermediate. The use of phosgene in the process results in the formation of HCl, which can cause degradation of the PEG backbone. Due to the volatile nature of phosgene, and the resulting safety and quality problems associated with its use, there is a need in the art for a method for preparing 1-benzotriazolylcarbonate esters of PEG without using phosgene.